1. Field of the Invention
This invention resides in the field of food processing, with particular interest in the processing of dried fruits.
2. Description of the Prior Art
The conventional process for drying grapes to make raisins entails field drying of the grapes in the sun. While generally effective and economical, field drying exposes the raisins to sand, dry leaves, and other particulate matter, and this exposure requires that the raisins be rinsed thoroughly before they can be packaged and sold. Proper cleaning is needed to meet standards imposed by the United States Department of Agriculture whose standard for Grade A raisins is no more than five units of sand per 250 grams of raisins and for Grade B raisins no more than six units per 250 grams, a unit being equal to 0.0588 cubic centimeters, as extracted and measured by a specified procedure. Commercial raisin buyers typically impose an even higher standard of either no sand at all or at most a trace amount. Wind exposure during the drying stage causes sand and dirt to be blown over the raisins, and heavy rain aggravates the problem since raindrops splashing onto the grapes carry even more sand with them. In both cases, the sand becomes embedded in the wrinkles that are formed when the grapes dry. Embedded sand is one of the most difficult particulates to remove and can be detrimental both to the economics of raisin production and to the raisins themselves.
Raisins with embedded sand are only one example of agricultural products that have wrinkled surfaces with undesirable particulate matter embedded in the wrinkles. Dried fruits in general are susceptible to clinging particles, and examples in addition to raisins are prunes, apricots, figs, peaches and cranberries. The embedded particles include not only sand and topsoil, but also in some cases shredded paper from coding stickers and other types of debris. The economic impact of all such debris is exemplified by the economics of raisin processing, which are a prominent example of the problem.
In raisin processing, conventional rinsing is often inadequate to remove the embedded sand, particularly when the sand is highly adherent or of a large quantity, or both. Removal of the sand can be achieved by puffing out the raisins in hot water, re-rinsing the puffed raisins, and then re-drying them in natural gas-fired dehydrators. The exposure to heated water in this process however tends to leach sugar from the raisins. This lessens the appeal of the raisins, and the puffing and re-rinsing steps add to the water consumption of the process and to the overall cost. Even with these extra processing steps, a measurable amount of the product is often lost, and the cost of a heavy rain to growers can amount to as much as 25-30% of the value of the crop.